CASH, CASH EQUIVALENTS AND INVESTMENTS - UNRESTRICTED AND RESTRICTED (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 301	$ 213
Investments	57	68
Restricted cash, cash equivalents and investments	1,388	1,180
Investments, noncurrent	138	125
Collateral for workers' compensation claims	210	212
Total	2,094	1,798
Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	1,228	1,018
Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	98	98
Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	60	62
Collateral for workers' compensation claims	210	212
Other security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	2	2
Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	301	213
Investments	0	0
Restricted cash, cash equivalents and investments	1,306	1,180
Investments, noncurrent	0	0
Total	1,643	1,456
Cash and cash equivalents \| Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	1,228	1,018
Cash and cash equivalents \| Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	16	98
Cash and cash equivalents \| Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	60	62
Collateral for workers' compensation claims	36	63
Cash and cash equivalents \| Other security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	2	2
Available for sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Investments	57	68
Restricted cash, cash equivalents and investments	82	0
Investments, noncurrent	138	125
Total	451	341
Available for sale marketable securities \| Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Available for sale marketable securities \| Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	82	0
Available for sale marketable securities \| Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Collateral for workers' compensation claims	174	148
Available for sale marketable securities \| Other security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	$ 0	0
Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents		0
Investments		0
Restricted cash, cash equivalents and investments		0
Investments, noncurrent		0
Total		1
Certificate of deposits \| Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments		0
Certificate of deposits \| Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments		0
Certificate of deposits \| Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments		0
Collateral for workers' compensation claims		1
Certificate of deposits \| Other security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments		$ 0

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as noncurrent.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26626-111562

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecuritiesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3044-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121566466&loc=d3e6676-107765

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_CashAndCashEquivalentsAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_RestrictedCashAndCashEquivalentsItemsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Cash and investments whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

+ References

No definition available.

+ Details

Name:	us-gaap_RestrictedCashAndInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The current cash, cash equivalents and investments that are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes current cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_RestrictedCashAndInvestmentsCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The noncurrent cash, cash equivalents and investments that is restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits classified as long-term; that is not expected to be released from such existing restrictions within one year of the balance sheet date or operating cycle, whichever is longer. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes noncurrent cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal.

+ References

No definition available.

+ Details

Name:	us-gaap_RestrictedCashAndInvestmentsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(4))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1)(g))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_ShortTermInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	tnet_CollateralbyTypeAxis=tnet_PayrollFundsCollectedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	tnet_CollateralbyTypeAxis=tnet_HealthBenefitClaimsCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	tnet_CollateralbyTypeAxis=tnet_WorkersCompensationClaimsCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	tnet_CollateralbyTypeAxis=tnet_InsuranceCarriersSecurityDepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CashAndCashEquivalentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CertificatesOfDepositMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: